Great Plane's recommendations on the 52 to 70 engine is because they don't want people to fly the U Can Do's too fast. They also rccommend a .32-50 2 stroke but a .32 2stroke probably won't even get it off the ground very well let alone do any kind of aerobatics with it. You can put a Saito 91 or 100 on the 46 Do and it will fly just fine but if you overspeed the plane even with a 55 OS AX you'll end up with a plane that can rip apart in the air. I've seen it happen. One of the guys at our field had a 46 Do with a 55 AX in it and was zipping around real fast with it one day and it just exploded and totaled out the plane. So that's why GP recommends the engines that they do with it.